1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus including a recording head for jetting liquid droplets.
2. Description of the Related Art
There are image forming apparatuses such as printers, fax machines, copiers, plotters, and multifunction peripherals including functions of these devices. An inkjet recording apparatus is known as an example of an image forming apparatus of a liquid jet recording method using recording heads constituted by liquid jetting heads (liquid droplet jetting heads) for jetting ink droplets. Such an image forming apparatus of a liquid jet recording method forms images (record and print may be used synonymously as form) by jetting ink droplets from recording heads onto a conveyed sheet (the sheet is not limited to a paper sheet; the sheet may be any sheet onto which ink droplets or other types of liquid can adhere such as an OHP transparency film; the sheet may also be referred to as a recording medium, a recording sheet, etc.). There are several types of image forming apparatuses that perform a liquid jet recording method. One example is a serial type image forming apparatus that forms images by jetting liquid droplets while moving the recording heads in a main scanning direction. Another example is a line type image forming apparatus that uses line type heads to form images by jetting liquid droplets while the recording heads do not move.
In the present application, an image forming apparatus that performs a liquid jet recording method means an apparatus that forms images by jetting liquid onto a medium such as paper, thread, fiber, cloth, leather, metal, plastic, glass, wood, ceramics, etc. Forming images on a medium means forming images having meaning (such as characters and figures) and forming images without any meaning (such as patterns, e.g., merely jetting liquid droplets onto a medium). Ink is not limited to what is generally referred to as ink; ink refers to any kind of liquid that can be used for forming images, such as recording liquid and fixing process liquid; examples of ink are DNA samples, resist, and pattern material. Furthermore, an image is not limited to a planar image, an image may be formed on a three-dimensional object, or a three-dimensional object may be formed.
There is known an image forming apparatus provided with a head tank for supplying ink to recording heads (also referred to as a sub tank and a buffer tank). In this type of image forming apparatus, ink is supplied from a main tank (also referred to as an ink cartridge), which is detachably attached to the main unit of the apparatus, to the head tank, with the use of a liquid sending pump constituted by a tube pump.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2010-143158
In a case where the above-described tube pump (tubing pump) is used for sending liquid, and ink is supplied from the main tank to the head tank, when the amount of ink remaining inside the main tank becomes small, the negative pressure inside the tube pump connected to the main tank increases. Thus, a difference arises between the pressure inside the tube pump and the atmospheric pressure outside the pump. Accordingly, air enters inside the tube through a flexible tube. When air enters the tube unexpectedly, the following problem arises when air is sent to the head tank.
Specifically, if the tube is filled with ink, and the main tank becomes empty (end state) and no more ink can be sent to the head tank, the end state of the main tank can be detected by detecting that ink is not being supplied at the head tank. Thus, a message prompting to replace the main tank is displayed and the operation can be stopped.
However, when the remaining amount of ink in the main tank becomes small (near end state), and air enters the tube before the ink end state, the air is sent into the head tank together with the ink. When a full state is detected at the head tank by detecting the displacement of a flexible member, the air mixed in the ink cannot be detected. Thus, even if the head tank is not sufficiently filled with ink, it will be erroneously detected that the operation of supplying (filling) the head tank with ink has been completed.
Consequently, even if there is no more ink remaining in the main tank, the ink end state cannot be detected, and therefore the operation of detecting the ink end state of the main tank is delayed. Thus, even more air enters the tube. Furthermore, foam is generated in the supply path as air and ink are mixed together. If this foam enters the head tank, and the head tank has a flow path connected to the atmosphere, the ink with foam enters the flow path connected to the atmosphere. Thus, when an atmosphere opening valve is opened, air is obstructed from properly exiting from/entering into the head tank.